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Irritrol 700-1 repair

I have several Irritrol 700-1 that are 20+ years old, two have stopped working and symptoms indicate that the diaphragm is leaking. I purchased two seal kits plus a new valve and am having some difficulty. The diaphragm seems too large for the valve base. There is a ridge around the edge of the diaphragm where it should fit into the groove of the base, but no matter how I try, it won't stay long enough to be able to reassemble the bonnet. After "playing" for almost an hour, I removed the bonnet of the new valve and discovered the same issue. I assume that Irritrol has a technique they use to assemble at the factory. I called Irritrol and was told to use some water to lubricate the diaphragm. It was obvious that the tech I spoke to had never seen the inside of a 700.

Has anyone repaired a 700 and what was the secret? I have tried most every 4 letter word I can think of.

I've rebuilt many 700 series valves. Both 3/4 and 1 inch.
I also have a box of old stock 700's. They're 1". The date code its from 1997. I just took one apart and reassembled it with no issue. The only thing I did that might be a hint was to insert the flat edge of the diaphragm first then I pressed the rest of the diaphragm into the groove. It fit snug and didn't move. The top went right on after that with no issue.
Make sure the diaphragm isn't upside down. The nipple in the center of the diaphragm should be face up.
I know they've redesigned at least one of the valves. It might be the 3/4 inch. But the new parts still work in the old valves.
There's a dramatic difference in the size of a 3/4" 700 and a 1" 700. So if you actually have 3/4" valves and are trying to put a 1" diaphragm inside there it will never work. I'm assuming that's not the case. The diaphragms look totally different anyway.

If there's water flowing out of the valve at all it can make assembling difficult.

You should make sure there's not a hairline crack in the valve seat. Imagine finally getting the valve assembled and having the same problem.
Take a close look at it with a flashlight.

I have reassembled the valve with the old diaphragm w/o a problem. The new one isn't as concave as the old one. I am going to chill it to see if it will assemble easier if it is stiffer. Has anyone tried a little (very little) Permatex in the groove...just enough to hold the diaphragm while installing the bonnet?